1. Field of the Invention
2.Background of the Disclosure
Three general types of protective coatings have been recognized in the optical cable art: 1) loose tube buffers, 2) tight buffers and 3) composite buffers. See Mahlke, G. and Gossing, P., Fiber Optic Cables, John Wiley and Sons Limited, N.Y., 1987, pp. 96-108. Other specialized constructions have also been developed. See, for example, Saito et al., U.S. Pat. No. 4,770,489, which discusses the use of a grooved spacer core to carry a group of fibers, wherein the grooves are filled with a soft jelly-like material, the spacer is surrounded with a layer of aramid fiber, and the aramid fiber is surrounded by a plastic jacket. Filling compounds and flooding compounds are used within cables to block the flow of water therein but they are not wholly interchangeable. Filling compounds are suitable for contact with fibers and must not include a base composition or additives that result in unacceptable levels of optical performance changes.
Flooding compounds, on the other hand, typically include ingredients that render them unsuitable for contact with optical fibers. Generally filling compounds will be located in a tube having at least one optical fiber, and flooding compounds will be located exteriorly of the tube.
In the loose tube construction, one or more fibers are surrounded by a protective tube, typically referred to as a core or buffer tube, which is spaced from the fiber(s) by a distance sufficient to allow the fiber(s) to move within the core or buffer tube in response to applied stresses. By placing the fiber loosely in a tube, the fiber is substantially decoupled from the rest of the cable. Typically, the space between the fiber(s) and the loose tube is filled with a filling material, e.g., a grease or a thixotropic gel, so as to inhibit or prevent water migration in the tube and/or to protect the fiber(s) from water absorption.
Greveling, U.S. Pat. No. 4,763,982, discloses a cable comprising: 1) an outer jacket and 2) an inner tube composed of strength members, such as aramid fibers, embedded in resin carrier, the inner tube being filled with a thixotropic water blocking medium. Peacock, U.S. Pat. No. 4,822,133, discloses the use of water blocking grease used as a filling material in a tube. Other filling materials have included petroleum jelly mixed with a polymer, usually polyethylene, as described by Biskeborn et al, U.S. Pat. Nos. 3,607,487 and 3,717,716. Other filling materials have included a low viscosity oil thickened by a styrene-isoprene-styrene copolymer, again with polyethylene added to impart consistency and reduce slump; see, for example, Dobbin et al., U.S. Pat. No. 3,879,575.
Sabia, U.S. Pat. No. 4,464,013, describes the use of a mixture of (1) from 85 to 94 percent by weight of ASTM Type 103, 104A, or 104B, or mixtures thereof, naphthenic or paraffinic oil having a minimum specific gravity of 0.855; a minimum SUS viscosity at 210° F. of 40; a maximum pour point ASTMD 97 of less than −5° F.; and a maximum of 5 percent aromatic oils; and (2) from 6 to 15 percent by weight of a styrene-ethylene butylene-styrene block copolymer having a styrene-rubber ratio of from approximately 0.2 to 0.5. These two ingredients comprise substantially 100 percent of the filling material, but with up to 1 weight percent of the filling material being stabilizer.
Patel, U.S. Pat. No. 4,497,538, proposes the use of a mixture of a block copolymer, petroleum and an additive. The block copolymer is a styrene-ethylene butylene-styrene (SEBS) having a styrene to rubber ratio of 0.39 to 0.41, and a specific gravity of approximately 0.91. The petroleum is a mixture of microcrystalline waxes and oil that contains a small amount of an antioxidant additive. The additive is a low molecular weight polyethylene having a molecular weight range from 1,000 to 10,000 and a specific gravity of at least 0.90.
A major component of the foregoing filling compositions is a petroleum based filling composition, for example, an oil, grease, or wax that has been mixed with minor amounts of one or more organic and/or inorganic compounds. Such petroleum based filling compositions are relatively expensive, require special blending and are not suitable for use with all cable components. For example, not all plastics commonly used for cable components are adequately resistant to the petroleum-based composition.
Another disadvantage of petroleum based filling compositions is that they may not be suitable in all environments. For example, a petroleum based composition may not form a network of molecular bonds that sufficiently reduces flow under normal cable use environments. As a result, petroleum based compositions are often messy, particularly for those working with the cable in the field. Moreover, known filling compositions are susceptible to dripping at high temperatures, and may cause undesirably high optical performance losses at lower operating temperatures.